The present invention relates to a sound synthesizing circuit, and particularly to a circuit for synthesizing a musical sound, for example the sound of a piano, a violin, a flute, a whistle, or the like.
In general, conventional musical sound synthesizers are very expensive because of the need for a large number of sound oscillators. However, by employing a semiconductor techniques, a low cost digital synthesizer has been provided in recent years. This prior art digital synthesizer uses a sound waveform synthesizing method which is used in a conventional speech synthesizer. In this synthesizer a waveform of a musical sound to be synthesized is sampled a plurality of times along a time axis, and the amplitude level at each sampling point is digitized according to a predetermined bit number. Thus, waveform data is stored in a memory and is read out of the memory when the musical sound is to be synthesized. In a musical sound waveform, the same waveform is repeated for all pitch periods. Therefore, a representative waveform in only one pitch period usually is used repeatedly to synthesize a desired musical sound. Thus, since memory capacity can be reduced, an integrated circuit on a semiconductor chip is realizable.
The waveform data stored in the memory are sequentially read out of the memory and are synthesized in a synthesizing circuit. The synthesized digital data is sequentially sent to a digital-to-analog converter and is converted to an analog signal in response to a sampling pulse. Thereafter, the analog signal is provided to a speaker.
However, the digital synthesizer of the prior art synthesizes a musical sound according to a sampling pulse which is generated at a predetermined fixed interval. That is, the digital to analog conversion is operated at the fixed same interval as the sampling pulse which is generated at an equal time interval. The sampling pulse is produced by dividing a clock signal generated by a clock generator.
On the other hand, the pitch period must be changed in order to synthesize a different tone from the fundamental tone stored in the memory. In other words, a frequency of the representative waveform has to be changed. The change of the pitch period can be performed by changing the interval of the sampling pulse. As described above, the sampling pulse is produced by a frequency divider which divides the clock signal by a dividing ratio corresponding to a tone to be synthesized.
However, in the case where a waveform in one pitch is divided into 32 sampling points, the following equations are to be satisfied: EQU Ts=N.times.T.sub.1 ( 1) EQU Tp=32.times.Ts=32.times.N.times.T.sub.1 (2)
Ts: a period of a sampling pulse PA1 N: a dividing ratio PA1 T.sub.1 : a period of a clock signal PA1 Tp: a pitch period of the waveform to be synthesized
As clear in the above equations (1) and (2), the pitch period Tp of the music sound waveform to be synthesized is changed by only the unit of 32.times.T.sub.1, if the dividing ratio N is changed by 1. Consequently, the number of tones which can be synthesized by the prior art synthesizer employing a waveform synthesis method has been limited to a small number. Further, when a desired tone frequency (pitch frequency) does not correspond to the integer magnifications of 32.times.T.sub.1, a frequency lag will be occurred. This lag is a maximum 16.times.T.sub.1 in one pitch period and is very offensive to the ear.
In order to reduce the frequency lag, the number of divisions of a waveform may be decreased. However, the decrease of the division number introduces inaccuracies into a musical sound with a complex waveform, such as a piano, a trumpet, etc. For these sounds a division number higher than a predetermined number is required.
Therefore, the prior art sound synthesizer has used a high clock frequency. However, an operating speed of an integrated circuit on a semiconductor chip must be increased according to the high clock frequency utilized. As a result, circuit design and also fabrication become very complex, and chip size becomes large. Moreover, in a semiconductor circuit having a complementary type transistor pair (CMOS) power consumption is in proportion to the square of the operating frequency (clock frequency), so that increasing clock frequency is unsuitable for the musical sound synthesizer including an integrated circuit.